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The DNSChanger Working Group's replacement DNS servers were taken offline as scheduled on Monday, 9 July.

However, rather than leaving an estimated 300,000 machines without internet services it seems that many ISP have configured their own substitute DNS servers, so that at least some pox-ridden machines still have a safety net.

What this means is that "infection count continues to decrease without a major crisis in support calls," according to a blog post by net security firm F-Secure. The Finnish security firm fielded three DNSChanger support queries of its own on Monday.

DNSChanger screwed the domain name system (DNS) settings of infected machines, redirecting surfers to dodgy websites as part of a long-running cybercrime. The FBI dismantled the botnet's command-and-control infrastructure back in November, as part of Operation GhostClick.

The takedown would have left compromised Windows machines without the ability to reach services that resolved domain names into IP addresses, leaving them effectively cut off from the net. A court order, twice extended, allowed the Feds to set up replacement DNS Servers. This provision was allowed to die off on Monday, but even after months in which to act – and more latterly warnings from Google and Facebook – a minority of machines remained infected.

More than four million Windows PCs were infected by DNSChanger. This figure has dropped to below 270,000 or lower over the weekend (estimates vary).

Sean Sullivan, security advisor at F-Secure said: "According to the latest IP count, the number of affected users in the UK has dropped to just 13,832, down from 19,589 on June 11. Clearly, the publicity surrounding the deadline has helped to raise awareness and the message is getting through that users need to clean up their computers.

"What we have seen is that some large Internet Service Providers have set up their own substitute DNS servers so their customers can stay online," Sullivan said. "So, despite the FBI being out of the game, that doesn’t necessarily mean that customers will be cut-off. It won't solve the problem completely though, and these users will find that they can't access the internet on their laptops from a Wi-Fi hotspot or friend's house."

Clean-up instructions and more background on the history of the malware and the operation that dismantled the huge cybercrime operation that exploited it can be found on the DNSChanger Working Group website here. ®